Enhancements to the login and registration forms

Description

I would like to propose a significant overhaul of the registration and login forms for 3.2, which I'd be happy to take on. A few things that I think would be helpful:

Combine registration processes for multisite and single-user into one form on one page.
Allow the registration and login to be templatized — no reason this should be for MS only.
Standardize filters, actions and variables between MS and single-user when at all possible.

My primary goal is to standardize the signup process between single-site and MS. A few things to consider:

The registration process for MS and single-site are different. In MS, you must activate your email address, in single-site you don't have to. I propose getting rid of the activation email and instead activating the user on first login. Also, I think the default text for the registration email can be finessed a bit and standardized across MS and single-site — right now on single-site it doesn't say anything.

I think the username restrictions should be standardized across both as well. More lenient by default seems better. Maybe a filter (on both) to allow people to define illegal characters?

The filters and actions between adding fields to MS and single-site registration pages should definitely be standardized. What's going to be harder is standardizing the filters and actions when validating and inserting a signup, because MS stores those signups in a different table until they're validated.

Q: Is that really necessary? Can we put all signups in the user table and purge unactivated ones on a semiregular basis? Seems like a lot of extra steps to have two tables devoted to this. And it would go a long way to standardizing that process.

I dunno... I can think of a few scenarios where dropping the login box into a main content area would not look very good. It also kind of looks like you got logged out of your site while reading it. Might rather leave the templating for a plugin?

If this moves forward, I'm happy to write new email text, but it should probably be on a separate ticket (one per issue so we can close them independently).

@Jane Can I recommend, then, a plugin to return the old login screen? If the login and registration screens are fully templatized then it's a simple matter to style it any way you'd like, including how it previously was styled, but right now everyone's hands are tied to do anything. The login and registration screens need significant work anyway, so this seems like an apt time to go all-out.

I dunno... I can think of a few scenarios where dropping the login box into a main content area would not look very good. It also kind of looks like you got logged out of your site while reading it. Might rather leave the templating for a plugin?

I think that with the advent of the toolbar, it's pretty obvious if you're logged in or not. In fact it would be nicer than now, where if I go to wp-login.php, logged in or not, it always prompts me to log back in. Which is related to #14949 and a different issue.

Making the signup page filterable would help a lot of plugins, including ones that limit how many sites you can create.